Nanoparticles offer interesting and frequently unexpected properties because their properties are rather the result of the surface of the particles than of the bulk volume. For example, nanoparticles can show surprising mechanical, optical and electrical properties, even at low concentrations. The properties of nanoparticles have attracted interest in polymer science, particularly for polymer reinforcement. Particular attention has been focused on carbon nanotubes (CNTs).
An important factor in polymer reinforcement is the nanoparticles' distribution in the polymer. It is believed that the nanoparticles, and in particular carbon nanotubes, should ideally be uniformly distributed in the polymer and each nanoparticle individually coated with the polymer so that an efficient load transfer to the nanoparticles can be achieved. Lack of homogeneity, i.e. uneven distribution of the nanoparticles, may create weak spots and an uneven distribution of stress, in consequence leading at best to only marginal increases in mechanical properties.
Due to difficulties in dispersion, the hopes of drastically improving the polymer's mechanical properties by the incorporation of nanoparticles have not yet been fulfilled. Hence, the need to improve the distribution of nanoparticles in polymers remains. Several processes are presented in the literature to obtain a good dispersion of CNTs in polymers, and polyolefins in particular, such as in-situ polymerization and use of a compatibilizer.
In-situ polymerization: in this approach, the CNTs are used as catalyst support, replacing more classical supports like silica or alumina phosphate. After impregnation or deposit of the catalyst on the CNT—support, the supported catalyst can then be introduced in a polymerization reactor. At the end of the polymerization, catalyst residues (containing the CNTs) can be well dispersed in the polymer matrix. A main drawback of this approach is the need to introduce the CNT—supported catalyst in an industrial polymerization reactor. Indeed, at the end of the CNT—polyolefin production, a complete cleaning of the reactor may be required. Such cleaning can be severe for safety reasons (due to the uncertainties on the CNT toxicity, there should be no contamination of the CNTs in the next polyolefin production) and for product quality reasons (any remaining traces of CNT in the reactor could induce a significant color change of the polymer prepared in the next run).
Use of a compatibilizer: numerous products are known to be interesting compatibilizers for CNT—polymer blends. Unfortunately, effective compatibilizers can correspond to quite expensive products. The final cost of the CNT/compatibilizer/polymer can be significantly increased due to the presence of such a compatibilizer.
There remains a need to provide nanocomposites with improved dispersion of the nanoparticles. There also remains a need to provide processes to obtain nanocomposites with improved dispersion of the nanoparticles.